16.6 Indent Tabs Mode

By default, Emacs inserts tabs in place of multiple spaces when it
formats a region. (For example, you might indent many lines of text
all at once with the indent-region command.) Tabs look fine on
a terminal or with ordinary printing, but they produce badly indented
output when you use TeX or Texinfo since TeX ignores tabs.

The following turns off Indent Tabs mode:

;;; Prevent Extraneous Tabs
(setq-default indent-tabs-mode nil)

Note that this line uses setq-default rather than the
setq command that we have seen before. The setq-default
command sets values only in buffers that do not have their own local
values for the variable.